The present invention is described below with reference to circular saws. However, this reference is not intended to limit the scope of the invention in any manner; the present invention can be used with any type of guide rail and any power tool used therewith.
Hand operated electrically powered cutting tools are typically used to cut a work-piece, such as wood panels or the like, into a desired shape or dimension. Often, the cut made by the tool needs to be straight in order to fit against or abut another straight surface. A guide rail can be used to guide the power tool along a straight edge of the rail, thereby providing a straight cut.
Typically, a guide rail consists of a piece of extruded aluminium cut to a desired length of roughly a meter or so. A rib runs along the length of the rail and the rib is arranged to cooperate with a channel formed on a base plate of a circular saw. The rib is arranged parallel to the edge of the rail along which a cut is to be made and the distance between the rib and that edge corresponds to the distance between a saw blade of a circular saw and the channel in the saw's base plate, for instance. Thus, when the saw is placed on a rail, the rib of the rail guides the saw blade along an edge of the rail.
A professional end-user, such as a cabinet maker or the like, often needs to make a cut in a work-piece at a predetermined angle with respect to a datum on the work-piece. The datum is typically an edge of the work-piece (which might have been pre-cut by the workman). As a result, a means for setting an angle of the guide rail with respect to the datum is required.
Such an arrangement is known from U.S. Pat. No. 6,725,558 which describes an adjustable portable power tool guide having a base plate, a base plate cutting edge to define a cut line, an adjustable tool guide rail for guiding a portable power tool cutter adjacent to the cutting edge, and an adjustable control bar that defines a reliable and predetermined angle between the cutting edge and the control bar. However, this system is relatively complex and the integrated control bar results in the guide rail only being usable when the end of the rail incorporating the control bar overhangs an edge of the work-piece.
In U.S. Pat. No. 6,708,422 a saw guide for guiding hand-held power saws is described. The saw guide includes a fixed arm of L-shaped cross-section comprising respective horizontal and vertical legs. The horizontal leg rests on the upper surface of the material with the vertical leg extending downwardly to engage an edge of the material. An elongate variable angle arm also of L-shaped cross-section comprising respective horizontal and vertical legs is pivotally connected to the fixed arm by an adjustable angle locking assembly. The horizontal leg of the variable angle leg rests on the surface of the material with the vertical leg extending upwardly to form an elongate, straight saw guide surface for guiding the power saw. The adjustable angle locking assembly pivotally interconnects the fixed arm and the variable angle arm for rotation about a common pivot axis. The locking assembly is lockable with the fixed arm and the variable angle arm in a plurality of angular positions indicated on an angle indicia
U.S. Pat. No. 4,608,761 describes a portable precision guide for powered hand tools having a base plate with a protractor portion and a table portion, a guide bar on one surface of the base plate guides a powered hand tool on the table portion and an angle arm pivotally mounted on the other surface of the base plate can be positioned by said protractor at a desired angle with said guide bar to position a work-piece for cutting.
EP 283553 describes a work bench for work-pieces to be worked by means of a hand tool such as a portable circular saw which contains a rectangular bench plate whose bench surface forms a mounting surface for the work-piece. Extending across the mounting surface is a guide rail which is held at a distance above the bench surface by means of supporting devices fastened to the bench plate in the area of the side surface.
U.S. Pat. No. 4,539,881 describes a guide plate for a hand power saw which has a top side with a low coefficient of friction for functioning with a guide member on the power saw, whereas the other or lower side of the plate has a slip-proof facing material for resting on a work-piece to be cut with the saw. Furthermore near one of its long edges the guide plate has a guide ridge with guide faces thereon running in the direction of sawing. A main part of the guide plate is placed on the one side of this ridge while the part on the other side of it is in the form of a ledge that is in a single plane with the lower face of the guide plate on the other side of the ridge. The lower side of the guide ridge is in this plane or is higher up than this plane. On the side of the main part of the guide plate furthest from the ridge there is a guide lip running in the direction of sawing that rests on the work-piece to be cut. This guide lip is made of non-splitting, elastic material running out from the side of the guide plate edge furthest from the guide ridge and the outermost edge of the lip is designed to be used as a cut marker.
One of the many problems identified with the prior art is the relative inaccuracy of the angular displacement of the rail to a datum on the work-piece. In particular, ensuring that a desired angular displacement between the rail and datum on a work-piece can be reproduced over various settings of different angles is problematic with prior art devices. The prior art devices are relatively complex where attempts have been made to overcome this problem.
Embodiments of the present invention aim to overcome problems associated with the prior art. Broadly, the present invention aims to provide a guide rail attachment which can be used to set accurately an angular displacement between a work-piece datum and the rail edge along which a cut is to be made. Furthermore, the present invention aims to provide an attachment which can easily be removed from a guide rail when it is not required by the end-user (for instance if the end user needs to make a straight cut but does not require an accurate angular alignment of the rail on a work-piece) and reattached with a high degree of repeatable accuracy.